English: Schematic illustration of the features most relevant to the fission phenomenon. The red curve depicts (in a one-dimensional projection) the potential energy as a function of the elongation; the ground state is at the lowest minimum, and the shape-isomeric state is at the second minimum. From these states it is possible to tunnel through the potential barrier. Tunnelling is also relevant for neutron or photon induced fission when the resulting initial state lies below the fission barrier. If the initial state is excited above the fission barrier, it may undergo a complicated shape evolution crossing the barrier from above. Once the system finds itself beyond the barrier, it relatively quickly descends towards scission. There it divides into two nascent fragments, which subsequently move apart under the influence of their mutual Coulomb repulsion while gradually attaining their equilibrium shapes and become primary fragments. Primary fragments then de-excite by evaporating neutrons, radiating photons, and undergoing β decay.